Primary 2 · Mathematics
P2 money
In cents, $9.05 is 905¢. The answer that often comes back is 95¢, because the 9 and the 5 got read as two separate numbers and the zero between them fell out.
- Level
- P2
- Usually taught
- Term 4
- Covers
- 1 concept
What changes at P2
Primary 2 writes money with a decimal point, as in $9.05, and converts between dollars and cents: $1 is 100¢. Your child counts notes and coins, finds the change from a note, and works out how much more one thing costs than another.
The point is new, and it hides a rule: there are always two places after it. $9.05 has a zero in the tens of cents, and that zero is easy to lose.
The sentence that costs the mark
Write $9.05 in cents.
What the working usually does
$9.05 in cents:9 and 5=95¢
Reads the two sides of the point as two numbers and puts them together.
What the question asked
$9.05 in cents:900¢ + 5¢=905¢
Changes the 9 dollars to 900 cents, then adds the 5 cents.
Both digits are there, in the right order, and 95¢ even looks like a sensible amount. Only remembering that one dollar is 100 cents shows that 95¢ is less than one dollar, let alone nine.
| Group | Detail |
|---|---|
| 1×$5, 2×$2, 1×5c |
The same question, worked properly
Split at the point
$9.05 = 9 dollars and 5 centsThe number before the point is dollars. The two places after it are cents, so .05 is 5 cents.
Change the dollars to cents
$1 = 100¢, so $9 = 900¢This is the step that fails. Each dollar is 100 cents, so 9 dollars is 900 cents, not 9 and not 90.
Add the cents
900¢ + 5¢ = 905¢$9.05 is 905¢. Going back, 905¢ is 9 dollars and 5 cents, written $9.05 with two places after the point.
How can you tell it has clicked?
Write $9.05 and ask how many cents that is. Then try $4.30 and $12.08. The amounts with a zero in them are the ones that show whether the 100 is understood.
Watch the other direction as well. 905¢ written as $9.5 means "two places for cents" has not stuck yet.
One thing to try tonight
Put nine $1 coins and a 5-cent coin on the table and ask how many cents that is.
Then swap one $1 coin for ten 10-cent coins and count them: 100 cents for one dollar. After that, nine dollars being 900 cents is something your child has counted, not been told.
What your child will meet in this topic
- Money
Most Singapore schools introduce this in Term 4, though schools do differ by a term either way. If your child’s school runs a different order, you can set it in the schedule on your dashboard.
What it leads to
P3 adds and subtracts money and works out change, and P4 meets decimals as numbers in their own right. Two places after the point for cents is the first decimal rule your child learns.
The same slip turns up in P4 decimals as 0.5 against 0.45. Both come from reading the digits after the point as a whole number.
The same topic in other years
It comes back each year and gets harder, so it helps to see what came before and what is coming.
More in Primary 2
Other topics in the same year, so you can see what sits either side of this one.
See how your child is doing on this topic
StudyLah marks each step separately, so you can see whether it was the setup or the arithmetic that cost the mark.